Whether a word lives or dies depends on laws of natural selection much like those that shape the fate of living species, a new study shows. Researchers analyzed more than 5 million books, written over the last two centuries in English, Spanish, and Hebrew, that had been scanned into Google’s vast database.

They found that while the English language is still growing at a rate of about 8,500 words per year, the birth rate of new words is slowing—and the death rate increasing. Newly coined words tend to achieve widespread circulation faster than they used to, because they are more likely to describe major innovations such as “Twitter” and “iPod.”

Meanwhile, in the “inherently competitive, evolutionary environment” of languages, dying terms are losing a Darwinian battle against more popular “synonyms, variant spellings, and related words,” study author Joel Tenenbaum, a Boston University physicist, tells The Wall Street Journal. The catchier term “X-ray,” for instance, put its synonym “roentgenogram” out of business; “persistency” has been choked off by “persistence.” Once a word is born, Tenenbaum found, it has between 30 and 50 years to either take hold or disappear.